This invention relates generally to thermoelectric devices and, more particularly, to methods for manufacturing thin film thermoelectric devices having high voltage densities at any temperature range of operation.
Over the past few years there has been substantial activity in the field of thin film thermoelectric devices, especially for use in electrical generator applications. Attempts to construct such generators using metallic thermocouples have not proven successful, however, due to the very low power of even the more favorable couples, e.g., bismuth and antimony, which generate not more than 0.1 mV per degree Centigrade. Therefore, semiconductor materials which possess high thermoelectric power, high electrical conductivity, but low thermal conductivity have been extensively used.
In the past, thin film thermocouples, i.e., thermocouples formed of layers of thermoelements ranging in thickness from 50 mils to fractions of a mil, have been manufactured by various techniques, such as machining from bulk materials by slicing and grinding, etching, sputtering and vapor deposition. In general, the thermoelectric elements are deposited directly on the insulating materials and the electrical contacts provided to form the couples. Compacting and sintering powdered thermoelectric elements has been suggested but this method cannot be applied to the formation of thin film thermocouples. See U.S. Pat. No. 3,086,068 granted to Charland et al. on Apr. 16, 1963. However, these techniques have proven to be somewhat less than satisfactory. Machined thermoelements cannot be produced in quantity in very small dimensions because the thermoelectric elements are inherently structurally weak. Vapor deposition and related techniques are difficult to control and quantity production of devices using this method has not been successful. Thin film thermoelectric devices produced using these techniques often require additional support in order to retain their structural integrity since the thin films themselves are not self-supporting. Thus, a problem exists in the production of thermoelectric devices of uniform dimensions in production quantities.
An equally imposing obstacle exists to the manufacture of thin film thermocouples for use as electrical generators. As mentioned above, the thermoelectric power of metallic thermoelements is too low to permit their use in such application. The efficiency of a metallic thermocouple is no more than 1 percent. Thus, widespread attention has been focused on thermoelectric generators employing semiconductors as the thermoelements. However, even using those semiconductors which appear most promising, such as bismuth telluride (Bi.sub.2 Te.sub.3), thermoelectric power ratings of only about 0.25 mV per degree Centigrade and an efficiency of no more than about 7 percent have been achieved.